In centrifugal blowers, it is known that periodic noise is generated by interference between an impeller and a tongue. This periodic noise has blade-passing-frequency whose frequency is proportional to the rotational speed (N) and the number of blades (Z) of the impeller.
In one known method for suppressing such periodic noise, a large gap is formed between the outer diameter of the impeller and the tongue. However, this method deteriorates the aerodynamic performance of the centrifugal blower. Therefore, to maintain the aerodynamic performance, that is, the airflow rate, it is necessary to increase the rotational speed of the impeller. However, this results in a problem that the overall noise level (which corresponds to an integration value of noise at respective audible frequencies) of the centrifugal blower is deteriorated.
An example technique (refer to Patent Citation 1, for example) for solving the above problem was proposed in which the generation of periodic noise is suppressed by varying the timing of interference between the tongue and the impeller by varying the ratio (tongue gap ratio s/D2) between the diameter D2 of the impeller and the gap s between the tongue and the impeller in the height direction of the impeller.    Patent Citation 1: Japanese Unexamined Patent Application, Publication No. 2002-339899.